Fish That 'Walks' Holds Clues to Animal Evolution

Spiders are known for nabbing insects, but many species frequently go fishing too, and researchers have the photos to prove it.
A new study in the journal PLoS ONE documents fish-eating spiders all over the world. Most are semi-aquatic species that usually dwell at the fringes of shallow freshwater streams, ponds or swamps, keeping an eye out for a fish dinner.
The spider Dolomedes tenebrosus was photographed devouring a creek chub on the banks of Bullskin Creek near Brutus, Kentucky, according to lead author Martin Nyffeler, a zoologist and spider expert from the University of Basel. He told Discovery News that this spider is less than an inch long and is often found in the U.S. south.

Jason Butler, Lexington USA

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This fish-eating spider is huge. Its leg span alone is close to 7 inches. The spider lives near freshwater streams and rivers in Central and South America, but visitors likely won't see it during the day. "Adults are strictly nocturnal," Nyffeler explained.
In addition to fish, Trechalea sp. eats a diverse array of other critters, including insects, shrimp and frogs.
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Many fish-eating spiders such as this are capable of swimming, diving and walking on the water surface. They have powerful neurotoxins and enzymes that enable them to kill and digest fish, which often exceed them in size and weight.
"The finding of such a large diversity of spiders engaging in fish predation is novel," Nyffeler said. "Our evidence suggests that fish might be an occasional prey item of substantial nutritional importance."
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Dolomedes facetus . It's abundant in Oz, according to Nyffeler. He added that the spider is "known for its habit of occasionally catching goldfish and platies (another type of fish) in garden ponds in suburban Australia."
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Peter Liley, Moffat Beach, Queensland

View Caption+#5: An adult

Ancylometes sp. was photographed preying on fish near Samona Lodge in the Cuyabeno Wildlife Reserve of Ecuador. It is another nocturnal spider that hunts at the edge of bodies of water when the sun goes down. Nyffeler said that the spider can dive and remain underwater for up to 20 minutes. In addition to hunting fish, it seeks out tadpoles, frogs, toads and lizards for supper.
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Tim Wohlberg, Kelowna, BC Canada

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The photographer in this case snapped the spider as it ate fish on the bank of the Rio Maicuru in Pará State, Brazil. Nyffeler and colleague Bradley Pusey from the University of Western Australia suspect that the spider is a juvenile, since it was out hunting in the daylight even though the species is known to be nocturnal. This fish-eating spider also hunts insects, shrimp and frogs.

Jacques Jangoux, Belém, Brazil

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A fisherman was surprised to find this spider. He had just mis-cast his fishing line, with the bait landing just off the edge of a dock near Sebago Lake, Me. That's when the spider, according to Nyffeler, "scuttled out very quickly from underneath the dock attempting to attack the live bait fish."
He continued, "Such incidences might be considered as predation attempts since the spider is grabbing a living fish with the intention to kill and devour it. The fact that fish are attacked even outside the water shows the high propensity for such spiders to feed on fish."
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Jeffrey Hollis, East Haddam, Connecticut

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This spider typically inhabits moist forests, where they hunt at night at the edge of bodies of water. Ancylometes sp. is enormous for a spider, with a leg span of close to 8 inches. Like many other fish-eating spiders, it goes after a variety of prey, such as tadpoles, frogs, toads and lizards.
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Ed Germain, Sydney, Australia

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Based on the new study, naturally occurring fish predation by spiders has been reported from all continents with the exception of Antarctica. This spider was snapped near Samona Lodge, Cuyabeno Wildlife Reserve in Ecuador.
In order to catch prey, this and other fish-eating spiders will typically anchor their hind legs to a stone or a plant, with their front legs resting on the surface of the water, ready to ambush. The fish will then be dragged to a dry place before the feeding process can begin. The spider will usually chow down for several hours on its fish feast.

Ed Germain, Sydney, Australia

View Caption+#10: A mosquitofish bit the dust in this moment, when a

Dolomedes triton individual caught the fish on the edge of a small, slow-moving stream near Fayetteville, NC. The species is one of the most abundant North American fishing spiders with a strong affinity to water. It is common in the wetlands of Florida and neighboring states.
The Florida wetlands are ground zero for fishing spiders, which seem to love the habitat there. Arachnid fans hoping to spot and photograph such spiders would do well to look for them at this location.

The evolution of the ancient fish that switched from living in water to living on land about 400 million years ago is one of the most pivotal moments in the history of the animal kingdom. These first four-limbed animals, the so-called stem tetrapods, ultimately gave rise to amphibians, reptiles, birds and mammals, including the largest animals to ever live on the planet.

When fish started moving onto land, "the fossil record suggests there was a great deal of diversity among fish, and thus a lot of competition between the fish," said lead study author Emily Standen, an evolutionary and comparative biomechanist at the University of Ottawa in Canada. "One can imagine there was a pretty good drive for those fish that could to get out of that environment and make use of opportunities on land." [Video: Unusual Fish that Can Walk & Breathe Hold Clues to Animal Evolution]

But just how ancient fish made this shift to terrestrial life still remains largely a mystery. To learn more about what happened when the now-extinct fish tried living on land, scientists investigated the bichir (Polypterus senegalus), a modern African fish that has lungs for breathing air, and stubby fins it can use to pull itself along on land. The bichir possesses many traits similar to ones seen in fossils of stem tetrapods, the researchers said.

The scientists raised groups of juvenile bichir on land for eight months to see whether these fish differed in their anatomy and how they moved on land compared with bichir raised in the water. Researchers wanted to test how life on land might trigger changes in such fish.

"The number one difficulty we faced was how the heck to keep fish alive on land for months at a time," Standen told Live Science. "I designed and built an aquarium setup that had kept a few millimeters of water on its floor, enough to keep the fish moist. In addition, I used misters, like you see in the lettuce aisle at grocery stores to freshen the vegetables, to keep the fish moist and in a very humid environment that helped them survive."

The researchers discovered the bichir raised on land were dramatically different than those raised in water. The land-raised fish lifted their heads higher, held their fins closer to their bodies, took faster steps and undulated their tails less frequently and had fins that slipped less often than bichir raised in water. These land-based fish also underwent changes in their skeletons and musculature that likely paved the way for their changes in behavior. All in all, these alterations helped bichir move more effectively on land.

"I'm very surprised the fish survived so well on land," said Standen, who conducted this research while she was a postdoctoral researcher at McGill University in Montreal. "That was an initial gamble with this experiment — could the fish even be raised on land?"

These findings reveal the bichir is more plastic — that is, malleable — during its development than previously thought. This plasticity is what made this fish capable of growing up very differently depending on its environment.

Given the anatomical similarities found between the bichir and stem tetrapods, the researchers suggest the animals' common ancestor could have possessed the kind of plasticity seen in the bichir today. If so, "this raises the possibility that plasticity may have also existed in stem tetrapods to facilitate their transition to land," Standen said.

"Fish that had the plasticity to allow them to move out onto land benefited by removing themselves from a very competitive environment into a new habitat of plants and insects supplying shelter and food resources, free of major predation or competition," Standen added. Over time, traits permitted by such plasticity may have proven advantageous enough to evolve into permanent fixtures in these ancient animals, she said.

Uncovering evidence of whether or not stem tetrapods really displayed such plasticity is very challenging. "The best way to find such evidence is to unearth fossils of a single population of these stem tetrapod fishes and look for natural variation in it," study co-author Hans Larsson, a vertebrate paleontologist at McGill University, told Live Science. "If we can, we might be able to find fossils that showed this population demonstrated some degree of plasticity."

Future research can seek to uncover the genetic and developmental mechanisms underlying the plasticity of the bichir, Larsson said. The researchers also want to raise multiple generations of bichir on land "to see how far this plasticity goes, how consistent developmental changes are in the long run," Standen said.

Standen, Larsson and their colleague Trina Du detailed their findings in the Aug. 28 issue of the journal Nature.